As a part of a trend for miniaturizing wireless communication terminal, there is an attempt to set up an antenna inside a wireless communication terminal.
FIG. 1 shows a structure of a conventional built-in antenna having a meander line structure. As shown in the drawing, the conventional built-in antenna 10 includes a radiator 12 for radiating the Global Standard for Mobile Communication (GSM) band, which is 900 MHz, to use multi-bands; a radiator 14 for radiating the Digital Command Signal band (DCS), which is 1800 MHz; and a feed point 16 for supplying electromagnetic signals to the antenna in the upper-left part of the antenna. The antenna 10 is attached to the upper part of a printed circuit board (PCB) of the terminal and thus set up inside the terminal. Here, the PCB is used as a contact surface of the antenna 10.
Meanwhile, since the space for installing the antenna in the terminal is very small, the antenna should be miniaturized while maintaining its performance. Therefore, the feed point 16 is formed at one end of an edge of the antenna 10, as shown in FIG. 1, to obtain the maximum antenna resonance length out of a minimum-sized antenna.
Since the conventional antenna which is mounted on a bar-type or flip-type terminal has a relative wide and fixed contact surface, the internal space of the terminal is very small. Moreover, since the size of the contact surface with the antenna is changed according to whether the folder cover of the terminal is open or closed, the antenna characteristics are degraded seriously. In particular, if the terminal is closed, the antenna contact surface becomes very small. So, the antenna characteristics are degraded seriously in DCS and Personal Communication Service (PCS) band which have relatively short wavelength of 1800 MHz to 1900 MHz.
FIG. 2 shows H-Plane radiation patterns of a folder-type terminal with a built-in antenna having a conventional feed structure. The H-Plane radiation pattern is a significant standard for observing the non-directionality of an antenna.
As shown, when the folder cover of a folder-type terminal is closed, the antenna characteristics are degraded in the 1800 MHz. That is, the transmission and reception is almost impossible in one direction of the terminal, which is the direction of 90° in FIG. 2, and the transmission and reception rate is degraded by more than scores of percentage in the directions of 0° and 180°, too. The degradation characteristics of the antenna having the conventional feed structure becomes known more obviously by interpreting three-dimensional full-waves, which are presented in FIG. 3.
FIG. 3 shows a result of interpreting: three-dimensional full-waves of the folder-type terminal which includes a built-in antenna having the conventional feed structure in the 1800 MHz. As shown, the antenna having a feed point in the upper-left part generates serious null in one direction of a radiation pattern because surface current is distributed only part of the antenna asymmetrically.
As consumers prefer smaller terminals, a demand for a built-in antenna increases continuously. This calls for the development of a small built-in antenna that can support multi-band characteristics and has stable transmission/reception characteristics in a folder-type terminal having a small contact surface, regardless of the open/closed state of the folder-type terminal.
At present, provided are various frequency band services including GSM scheme which occupies about 80% of the world market and DCS scheme. Therefore, an antenna that can perform transmission/reception stably in multibands is required.
In order to satisfy the diverse needs of current customers, terminals are miniaturized more and more and the miniaturization of terminals calls for the development of a built-in antenna that can be mounted on a small terminal. Also, a small built-in antenna with stable transmission/reception characteristics even in a folder-type terminal, of which internal space is very small and contact surface is changed according to whether the folder cover is open or closed, is in a desperate need.